Oxygen transport membranes are devices that are formed from ceramics that exhibit oxygen ion conductivity at elevated temperatures. An oxygen containing feed, for instance air, is contacted on one surface of the membrane, known as the cathode side, and becomes ionized by gaining electrons. The oxygen ions are then transported through the membrane under the impetus of an oxygen partial pressure differential to an anode side in which the oxygen ions recombine and give up electrons to form elemental oxygen.
The ceramic material forming the oxygen transport membrane can be a mixed conductor in which both oxygen ions and electrons are conducted. The electrons produced at the anode side of the membrane by the formation of elemental oxygen and are then conducted to the cathode side of the membrane to ionize the oxygen within the oxygen containing feed. In ionic conductors the ceramic material making up the oxygen transport membrane is only capable of conducting the oxygen ions. As such a separate electrically conductive pathway must be provided for the electrons. Such oxygen transport membranes can also operate by drawing externally generated power through the separate electrically conductive pathway. In dual phase conductors, an ionic conductor and a conductive metal phase are combined for transport of both the oxygen ions and electrons.
As stated previously, oxygen transport membranes function at elevated temperatures, for instance between about 400° C. and about 1100° C. Typically extra heat must be supplied to oxygen transport membrane reactors to compensate for heat leak and cold end losses even when heat is recovered from product and waste streams. In larger plants this is provided by the combustion of fuel in in-line combustors or fired heaters. Typically, the combustor and oxygen transport membrane are separate units requiring separate high temperature containment vessels and several insulated high temperature pipe runs. Alternatively and/or in addition, reactive purges of a combustible reactant can be introduced to the anode side of the membrane to partially consume the permeated oxygen and thereby drive the separation while heating the oxygen transport membrane. In such case any remaining oxygen permeate contains combustion products.
As will be discussed, the present invention provides an oxygen transport membrane reactor and method in which there is no need for external piping runs and the like and the oxygen product is not contaminated by the combustion products.